Integration of an external IMAP

I plan to install a Zimbra OS ed, as multi server. I have just a couple of 100's users that actually reside on a regular Postfix/cyrrus server. (P/C)

In the begining I just need One datastore, and I would like to integrate an IMAP server (postfix/Cyrus) as if it was a datastore, then I could move the box from the OLD postfix/Cyrrus to Zimbra.

On Postfix/Cyrrus I can route using /etc/postfix/transport
On Zimbra I expect zimbraExternalImapHostname to route and config correctly NGINX so the main server will known were to route imap connections.
I'am not sure of that and I am testing this.

Of course there would be no webmail on the Old P/C, I just want to known if I can route IMAP to the OLD server while setting my new Zimbra installation, so the move will be easier.

however , i will for sure use the split domain to ensure the routing between both server, at MTA level.
my today problem is about client's access to the servers at IMAP level, because as of today all users are IMAP clients.

The solution I look for consist in a progressive move from the OLD IMAP server to the NEW ZIMBRA, without reconfiguration of all imaps clients.
But your definitively right Bill.